Clintons stand together over email controversy

Former US president Bill Clinton has defended his wife Hillary, the Democratic presidential frontrunner who has come under attack for using a private email server while serving as secretary of state.

The former president said he has "never seen so much extended on so little."

In an interview aired on Sunday on CNN the 42nd president pegged the scandal as overblown hype, saying such affairs have "been a regular feature of all of our presidential campaigns."

"The other party doesn't want to run against her. If they do, they'd like her as mangled up as possible," he said.

Hillary Clinton has been dogged for months by revelations that she used a private email account and home server in lieu of the official government email system while she was secretary of state from 2009 to 2013.

Clinton said his wife, who has seen her lead in national polls narrow, was the target of competitors attempting to whittle away her frontrunner status.

"They thought the only way they could make it a race was a full-scale frontal assault on her," Bill Clinton said. "And so this e-mail thing became the biggest story in the world."

"I actually am amazed that she's borne up under it as well as she has," he said.

The State Department, to which Hillary Clinton already turned over 30,000 official emails in late 2014, has publicly released thousands of them in the interest of transparency.

Many contain information that has been retroactively classified, raising questions about whether Clinton was inappropriately sending and receiving highly sensitive material, and whether sufficient security measures were in place to protect her server from hackers.

"At the beginning of the year, she was the most admired person in public life," Bill Clinton said.